The discovery hole reported intercepts of 393g/t gold over 2.2m, 31g/t over 6.1m and 82.9g/t over 2.3m.
Called Windfall North, the previously unexplored area is only 350m north of the main deposit. Mineralisation is characterised by gold-bearing quartz veins in a silica-carbonate-sericite alteration envelope, in places up to 6m wide.
Osisko encountered the mineralisation 50-275m below surface and it remains open at depth and along strike.
The exploration team is tracing two near-parallel zones following the same general trend of the main deposit, as it plunges at 45 degrees northeast.
CEO John Burzynski said he was confident the discovery was a "very positive" sign that there was good potential to add new ounces, near surface. "We expect to continue to grow the size of our deposit as we advance our work at Windfall, both near surface and with our deep drilling programme."
Windfall Lake currently hosts a high-grade resource of 2.87Mt grading 8.17g/t for 754,000oz in the indicated category, and 10.35Mt at 7.11g/t for 2.37Moz inferred.
Osisko recently reported rich infill results at Windfall, including 2,223g/t over 2m.
The Toronto-quoted stock closed at C$3.09 on Tuesday, up 8.42% for the day and taking year-to-date gains to 5.1%. The company has a market capitalisation of C$811.89 million.